This invention relates generally to the use of pressure-containing vessels to control the elevation and attitude of objects submerged in a body of liquid, especially in environments where hydrostatic crush is of significant concern. The objects may be connected to one or more pressure containing vessels, such as subsea jackets used to support wind turbines or oil rig platforms. The objects may be the pressure-containing vessels, such as pipelines.
In a presently known method of moving an object from one subsea location to another or raising and lowering an object between the surface and the seabed, small glass microspheres containing air or other gas are dispersed in a body of liquid to form a buoyant fluid. The fluid can be injected into or evacuated from a bladder which is disposed inside of a rigid housing. A valve allows seawater to be injected into or evacuated from a void in the housing and around the bladder. If the void is water filled, as the bladder is expanded or contracted by the addition or evacuation of fluid, water will be evacuated from or admitted into the void in the housing.
The depths at which the microsphere/bladder system can effectively operate are limited. The fluid is a dispersion of the gas-containing microspheres in a liquid and is, therefore, incompressible. The gas contained in the microspheres is the predominant source of system buoyancy. The wall thickness of each microsphere must be sufficient to withstand the proportionate-to-depth internal anti-hydrostatic pressure of the housing. Therefore, the operating depth is limited by the competing interests of pressure-withstanding microsphere wall thickness and the volume of buoyancy-providing gas in the microspheres.
The attitude of the housing of the microsphere/bladder system in the water cannot be controlled. When the bladder is not fully expanded, its shape, and therefore the distribution of gas in the housing, is unpredictable. Even if the bladder is fully expanded, there is no external structure guaranteeing that its position in the housing is constant. And, even if the bladder does initially assume its intended shape and location in the housing, if the distribution of gas in the bladder is uneven, the housing will experience unpredictable changes in shape and location during operation. The purpose of the glass microspheres is to assure that the buoyant gas they contain is evenly distributed in the liquid that is used to fill the bladder. Regardless of the orientation of the bladder in the housing, if sufficient microspheres are damaged or destroyed, perhaps by depth increases as explained above, their gas is freely dispersed into the liquid, and the stability of the system is compromised.
The ratios of buoyant fluid to water is not known throughout the operation of the microsphere/bladder system. The bladder changes its shape as the liquid containing the microspheres is added to or evacuated from the bladder, but the void between the bladder and the housing may never be fully evacuated of air or water. Therefore, while the amount of the liquid in the bladder may be controlled, the ratio of housing-contained liquid to water is not precisely known. Furthermore, because the microsphere/bladder system requires the bladder to expand to a substantial part of the total housing, use of an elongated housing, such as a pipeline, is impractical.
In present shallow water pipeline laying practices, buoyant primary pipelines are capped and floated to the laying site and, by controlled flooding with seawater, are submerged to the seabed while non-buoyant primary pipelines, perhaps containing one or more cables and/or other pipelines, have piggybacked secondary pipelines which are capped and provide sufficient buoyancy to float the combination. The primary pipeline, or primary and piggybacked secondary pipelines together, are floated to the laying site where the primary, or primary and/or secondary pipelines, can be control-flooded to provide the necessary ballast to submerge the primary pipeline.
The present applications of known shallow water pipeline laying practice are illustrated in FIG. 1. Each block of FIG. 1 shows the primary pipeline PP in its floated condition prior to controlled flooding with water to cause the primary pipeline PP to sink. Block 1 shows an empty primary pipeline PP which is filled with air A. Block 2 shows an empty primary pipeline PP which is filled with air A and is piggybacked to a secondary pipeline PS which is also filled with air A. Block 3 shows a primary pipeline PP which is filled with air A and contains a cable/other pipeline Z. Block 4 shows a primary pipeline PP which is filled with air A and contains a cable/other pipeline Z and is piggybacked to a secondary pipeline PS which is also filled with air A.
None of these FIG. 1 applications are useful in deep water applications. Without a piggybacked secondary pipeline PS, a primary pipeline PP heavy enough to sink to deep water levels cannot be floated. On the other hand, a secondary piggybacked pipeline PS satisfactory to provide flotation for a non-buoyant primary pipeline PP must have such thin walls that the secondary piggybacked pipeline PS will be destroyed by hydrostatic crush before deep water levels are reached. Therefore, the presently known shallow water flotation practices are effective to submerge pipelines only to depths in a range of 60 to 70 meters. If primary pipelines are intended to contain one or more cables and/or other pipelines, the weight of their contents must also be overcome, likely reducing the maximum depth that can be reached to less than 60 or 70 meters. If the air contained by the secondary pipeline were compressed, it might be possible to reach a depth of approximately 100 meters before hydrostatic crush occurs, but 100 meters is still relatively shallow for offshore pipelines.
In laying pipelines at greater depths, delivery is presently, and has for about a half century been, accomplished in one of two ways. In some applications, sticks of pipe are transported to a laying-site welding platform the pipeline is assembled offshore. In other applications, the pipeline is assembled on shore and plastically coiled onto a reel. The reel of coiled pipeline is transported to the laying site. The offshore-assembled or reel delivered pipeline is then laid on the seabed by known J-lay or S-lay techniques.
When delivering sticks of pipe to the site, the size of the delivery vessel is generally dictated by a comparison of its size and cost with the time and expense of the total number of trips required between the shore and the site to deliver all the sticks needed to construct the pipeline. When delivering reels of pipeline to the site, the number of trips is greatly reduced but the cost of the vessel increases exponentially.
Whether by pipeline flotation or reeled pipeline delivery, the cost of laying, for example, a 30″ diameter pipeline 1,500 meters in length in deep water typically ranges from $10,000,000 to $30,000,000. If the product pipeline is intended to contain one or more cables and/or other pipelines, the time and costs associated with the construction and/or the delivery of the pipeline off-shore are further exacerbated.
In sum, there are known object handling practices with depth and control limitations, known pipeline laying practices which can use crafts as small as tugboats but are limited to very shallow water applications and known pipeline laying practices for deep water applications which involve much larger ships and/or great time and expense and are still fraught with hydrostatic crush complications.
It is, therefore, a primary object of this invention to provide a method for controlling the elevation and attitude of pressure-containing vessels in a body of liquid. It is also an object of this invention to provide a method for controlling the elevation and attitude of objects connected to pressure-containing vessels in a body of liquid. It is another object of this invention to provide a method of delivering pipelines to and laying pipelines at offshore deep-water laying sites which is less costly and less time-consuming than known methods and which facilitates the present method of controlling the elevation and attitude of pressure controlling vessels and of objects connected to pressure-containing vessels. A further object of this invention is to provide a method for controlling the elevation and attitude of pressure controlling vessels and of objects connected to pressure-containing vessels which counteracts the varying forces of hydrostatic crush over great changes in depth.